A so-called two-component type heat-sensitive recording material using color forming reaction between a colorless or slightly colored electron donating dye precursor and an electron accepting compound is disclosed in Japanese Patent Publication Nos. 14039/70 (corresponding to U.S. Pat. No 3,539,375) and 4160/68 and so on. A two-component color forming type heat-sensitive recording material is prepared by dispersing a colorless or slightly colored electron donating dye precursor and an electron accepting compound into a fine particle state, mixing a binder and the like therewith so that these two heat-sensitive compounds are separated and coating the mixture on a support. One or both these heat-sensitive compounds are fused and contacted each other upon heating, resulting in a color forming reaction to conduct recording. Such two-component color forming type heat-sensitive recording material are advantageous in such points that: (1) primary coloration is conducted and therefore development is unnecessary; (2) paper quality is similar to that of a general paper; (3) handling is easy; (4) color density is high; and (5) upon color formation various hues can be obtained, and accordingly such recording material is very valuable. Therefore, the two-component color forming type heat-sensitive recording material has become widely used, particularly recently in the fields of facsimile, recorders, and printers. With the increased wide usage in the field of facsimile, recording rate is made higher and higher. And with this tendency, a heat-sensitive recording material has been strongly demanded to have short pulse, that is, color formation with low energy, that is, improvement of heat reactivity, has been strongly demanded.
On the other hand, the heat-sensitive recording materials typically have defects such as that an electron donating-dye precursor and an electron accepting compound are apt to react not only upon heating but also by action of solvents, because the recording materials utilize primary coloration.
These two compounds react in a solvent, because they are organic compounds and have high solubility to a solvent. For the above reason, white part of a heat-sensitive recording material is apt to form color when contacted with a writing material containing a solvent, such as an aqueous ink pen, an oily ink pen, a fluoroscent ink pen, a diazo developing solution or an adhesive, and printed letters of the heat-sensitive recording material are apt to disappear, resulting in largely decreased commercial value thereof.
In order to overcome the above problems, provision of a solvent-resistant protective layer on the heat-sensitive color forming layer has been proposed in the past. However, such a protective layer has not always been satisfactory because it is not sufficiently water-resistant, that is, the protective layer peels off from the heat-sensitive color forming layer or the protective layer itself dissolves when the heat-sensitive recording material is dipped in a water.